1. Field of the invention
The present invention relates to field controlled diodes (FCDs), and more particularly, to field controlled diodes having reduced gate current levels during turn-off.
2. Background Information
The field controlled diode is a PIN junction diode in which current flow in the reverse direction is blocked by the junction of the diode and in which current flow in the forward direction may be blocked by application of an appropriate voltage to a junction gate to pinch off the cathode-to-anode current path of the diode. When a forward bias voltage is applied across the anode/cathode circuit of the field controlled diode and the field controlled diode is held in the off state by the bias voltage applied to the junction gate electrode, there is substantially no current flow between the anode and cathode electrodes. However, a current which can vary between small and significant flows in the gate circuit because a reverse bias is applied to the gate junction to create the depletion region which pinches off the anode/cathode conductive path. This reverse biased junction collects any thermally generated carriers which would otherwise flow in the anode/cathode circuit. The level of this gate current depends on the diode's characteristics and the ambient temperature and radiation conditions, since thermally generated carriers and optically generated carriers both add to the gate current. In order to allow the diode to become conductive under these conditions, the applied gate bias voltage is changed to one which reduces the reverse bias on the gate junction to a level at which the anode/cathode current path is no longer pinched off. Current is then carried in the anode/cathode circuit in the normal manner for junction diodes. Consequently, when the diode is ON, a significant stored charge develops in the body of the diode.
When it is desired to turn the diode off by gate control, a gate bias voltage is applied to the gate electrode which reverse biases the gate junction and produces a depletion region associated with that junction which eventually pinches off the anode/cathode current path. Unfortunately, in order for that depletion region to be depleted of mobile carriers, a substantial portion of the stored charge in the body of the diode must cross the gate/diode junction to be collected by the gate region. This produces substantial gate currents which may be on the order of 50% of the ON-state current of the field controlled diode. Once the gate junction depletion region pinches off the anode/cathode path of the device and the stored charge has been dissipated, the gate current drops to the quiescent value in the manner discussed above.
Unfortunately, the requirement for dissipating the stored charge during the turn-off process for a field controlled diode results in a relatively long switching time for the field controlled diode and the dissipation of substantial power during the turn-off process. Further, the high gate current during turn-off, establishes the power handling capacity which is required in the gate control circuit and that circuit must therefore be capable of handling substantial power.
There is a need for a field controlled diode providing low gate current turn-off of the diode.